The invention relates to testing a coating on a conductor. In particular, the invention is a method and apparatus for detecting breakdown of a coating on a conductor, such as a wire.
Insulated wire, such as magnet wire, is produced in large commercial quantities from bare copper wire, aluminum wire, or another conductor, by applying a dielectric coating to the bare conductor. For example, a liquid resin enamel is applied to the bare conductor and cured by passing the enamel coated conductor through an oven. Plural coating layers can be applied to provide the desired insulating and/or dielectric properties of the coating. It is ideal that there be no flaws, or faults, or other breakdown in the dielectric coating. However, this is not readily achieved and thus manufacturers generally warrant that a particular wire has only a specified acceptable number, or less, of faults per unit length. To make this assurance, the manufacturer should test the wire along substantially its entire length. Various forms of wire have various insulative or dielectric coatings. The terms "insulator" and "insulative", as used herein, refer to any material or materials having a relatively low electrical conductivity with respect to an electrical conductor. The term "dielectric", as used herein, refers to any insulator which facilitates storage of energy in the form of an electric field. The term "breakdown", as used herein, refers to any defect, fault, damage, or other undesirable characteristic of the coating.
Since wire is produced in large quantities and handling these large quantities is difficult and expensive, wire is generally tested as it leaves the oven or as it leaves another processing step. Wire generally is transported at speeds up to several hundred feet per second during processing. Accordingly, it is desirable to test the coating while the wire is moving at high speed. It is known to place a high voltage across the outer surface of the coating and the conductor to test the coating for breakdown. For example, the wire can be run across a wheel coupled to the positive lead of a high voltage source (3000 VDC for example) while the conductor is coupled to ground. Current fluctuations are measured to detect coating breakdown. It is desirable to use a relatively high voltage to produce current flow for even incomplete breakdown. For example, for many wire applications, it may be necessary to detect when the resistivity of the coating is less than a threshold value which itself is quite high.
In order to accurately detect and count instances of coating breakdown, collect data relating to the counted instances, and use the collected data in reports and process quality control, it is desirable to interface the detection apparatus with modern data collection equipment, such as programmable controllers, personal computers, process controllers, and the like. Often such data collection apparatus is digital and operates at a logic level of 1 to 5 volts. Accordingly, it has been necessary to use bulky and expensive signal processing components to interface the fault detection apparatus, which operates at a high detection signal voltage, to data collection equipment, which operates a low voltage. For example, arrays of high voltage resistor networks have been used to divide the high detection signal voltage down to a level appropriate for data collection. For example, U.S. Pat. No. 3,413,541 discloses an apparatus for detecting insulation faults in magnet wire utilizing high power resistors and other components to divide a detection voltage for use in driving a recorder.
Therefore, conventional apparatus and methods for detecting dielectric breakdown require a large number of high voltage components, power resistors in particular, and thus are relatively large and expensive. Also, conventional apparatus and methods are not easily interfaced to digital data collection equipment and thus do not facilitate data collection and process control.